1. Field of the Invention
The present invention relates to a printed wiring board provided with a signal transmission pattern for transmitting a high frequency signal in which characteristic impedance becomes a problem, and a manufacturing method of the printed wiring board.
2. Description of the Related Art
In a dielectric substrate requiring high frequency and high speed digital transmission, a characteristic impedance is generally required for a transmission line (pattern). This characteristic impedance is an electric circuit constant expressing characteristics of the transmission line wired on the substrate for handling a high frequency current. The value of the characteristic impedance is set so that the transmission line is electromagnetically coupled to a solid power supply layer and a solid ground layer to form an equivalent distributed constant transmission line. In regard to the above request, a multilayer substrate is adopted, and etching widths of an image plane connected to a solid layer and an insulating layer of a transmission line are made to have desired values with precision, so that a printed wiring board (hereinafter referred to as xe2x80x9can impedance control boardxe2x80x9d) capable of controlling the characteristic impedance is realized.
FIGS. 8A to 8C are sectional views showing structural examples of conventional impedance control boards 110, 110a and 110b, respectively.
Values of these characteristic impedances are values which can be calculated by an approximate expression with parameters of a thickness H between prepregs 111, a width W of a transmission line 9, a thickness T of a solid ground layer 19, and a relative dielectric constant ∈. The precision of the characteristic impedance is determined by the precision of these parameters.
The impedance control boards 110 and 110a of multilayer substrates shown in FIGS. 8A and 8B are multilayer substrates of microstrip line structure and stripline structure, respectively.
On the other hand, the impedance control board 110b shown in FIG. 8C adopts a coplanar structure which is not a multilayer substrate, and has such a structure that a transmission line 9 having characteristic impedance to be controlled is sandwiched between solid ground layers 19 (hereinafter referred to as xe2x80x9cground linesxe2x80x9d). The impedance control board 110b can be realized by substantially the same constitution in either case where the transmission line 9 is formed on both sides or on one side.
However, since the impedance control board 110 and 110a shown in FIGS. 8A and 8B are respectively multilayer substrates, specific finishing precision and its management step are required, and there has been a problem that costs become very high. Besides, it is necessary that the multilayer substrate is generally manufactured by an expensive batch type large press and by taking labor, and there has been a problem that production efficiency is poor. Besides, when it is desired to control a characteristic impedance of only a part of the substrate, the multilayer plate must be used, and the cost has not been reasonable.
On the other hand, in the impedance control board 110b shown in FIG. 8C, in the case where a number of transmission lines 9 are wired to be parallel to each other like, for example, bus lines, the sandwiching ground lines 19 are required for each of the transmission lines 9, and there has been a problem that a nearly double wiring space is required and wiring design is hard to make.
Then, the present invention has an object to solve the above problems and to provide a printed wiring board in which control of a characteristic impedance of a signal transmission pattern is realized with a small space and at a low cost, and high manufacturing efficiency, a delay is hard to cause, and a high frequency signal can be stably transmitted.
According to a first aspect of the invention, the above object can be achieved by a printed wiring board characterized by comprising at least one signal transmission pattern with a predetermined width and thickness provided on a surface of a base material, for transmitting a high frequency signal; an insulator layer formed to a constant thickness on said surface of said signal transmission pattern for insulating said signal transmission pattern; a connection pattern, formed out side of said insulator layer and on said surface of said base material, and connected to a reference voltage; and a opposing pattern provided on a surface of said insulator layer so as to oppose said signal transmission pattern and to be connected to said connection pattern.
According to the constitution of the first aspect, the signal transmission pattern is provided on at least one surface of the flat plate base material, and has a function of transmitting a high frequency signal. This signal transmission pattern is covered with the insulator layer formed to the constant thickness. A connection pattern is formed out side of said insulator layer and on said surface of said base material, and connected to a reference voltage. On the other hand, an opposing pattern is provided on the insulator layer so as to be opposite to the signal transmission pattern. This opposing pattern has a function of controlling the characteristic impedance of the signal transmission pattern. Thus, the characteristic impedance is controlled and the high frequency signal of the signal transmission pattern is stably transmitted without causing a delay. Besides, when the opposing pattern is connected to a reference voltage via the connection pattern, unnecessary radiation noise radiated from other surrounding signal transmission patterns or the like can also be cut off.
The invention of a second aspect is characterized in that in the constitution of the first aspect, the opposing pattern is formed of a conductive paste.
According to the constitution of the second aspect, in addition to the operation of the first aspect, the opposing pattern can be easily formed on the insulator layer even if a multilayer substrate is not used.
The invention of a third aspect is characterized in that in the constitution of the first aspect, a width of the opposing pattern is three or more times as large as a width of said signal transmission pattern.
According to the constitution of the third aspect, in addition to the operation of the first aspect, when the width of the opposing pattern is on this level, it becomes sufficiently wide relative to the width of the signal transmission pattern. Accordingly, the opposing pattern can suitably control the characteristic impedance of the signal transmission pattern.
The invention of a fourth aspect is characterized in that in the constitution of the first aspect, the insulator layer is formed of a dry film of an epoxy resin having photosensitivity.
The invention of a fifth aspect is characterized in that in the constitution of the first aspect, the insulator layer is formed by curtain coat coating of a liquid epoxy resin having photosensitivity.
The invention of a sixth aspect is characterized in that in the constitution of the first aspect, the insulator layer is formed by printing using a photo-curing resist.
The invention of a seventh aspect is characterized in that in the constitution of the first aspect, the insulator layer is formed by printing using a thermosetting resist.
According to the respective constitutions of the fourth to seventh aspects, in addition to the operation of the first aspect, the insulator layer can be easily formed so as to cover the signal transmission pattern.
The invention of an eighth aspect is characterized in that in the constitution of the first aspect, the opposing pattern is connected to a ground layer provided on the other surface of the base material through a via hole for conducting electricity to pass through both the plane surfaces of the base material.
The invention of a ninth aspect is characterized in that in the constitution of the first aspect, the opposing pattern covers a part of the signal transmission pattern formed on the surface of the base material.
According to the invention of a ninth aspect, the above object is achieved by a manufacturing method of a printed wiring board, characterized by comprising at least one signal transmission pattern with a predetermined width and thickness provided on a surface of a base material, for transmitting a high frequency signal; an insulator layer formed to a constant thickness on said surface of said signal transmission pattern for insulating said signal transmission pattern; a connection pattern, formed out side of said insulator layer and on said surface of said base material, and connected to a reference voltage; and a opposing pattern provided on a surface of said insulator layer so as to oppose said signal transmission pattern and to be connected to said connection pattern.
According to the constitution of the ninth aspect, the signal transmission pattern and the connection pattern are provided on a surface of the flat plate base material, and has a function of transmitting a high frequency signal. This signal transmission pattern is covered with the insulator layer formed to the constant thickness on the other hand, the opposing pattern is grounded via the connection pattern and is provided on the insulator layer so as to be opposite to the signal transmission pattern. This opposing pattern has a function of controlling the characteristic impedance of the signal transmission pattern. Thus, the characteristic impedance is controlled by the opposing pattern and the high frequency signal of the signal transmission pattern is stably transmitted without causing a delay. Besides, when the opposing pattern is connected to the reference voltage, unnecessary radiation noise radiated from other surrounding signal transmission patterns or the like can also be cut off. According to the manufacturing method of the printed wiring board like this, the printed wiring board can be manufactured by fewer steps than multi layer substrate board.
The invention of a tenth aspect is characterized in that in the constitution of the ninth aspect, a width of the opposing pattern is three or more times as large as a width of the signal transmission pattern.
According to the constitution of the tenth aspect, in addition to the operation of the ninth aspect, when the width of the opposing pattern is on this level, it becomes sufficiently wide relative to the width of the signal transmission pattern. Accordingly, the opposing pattern can suitably control the characteristic impedance of the signal transmission pattern.
The invention of a eleventh aspect is characterized in that in the constitution of the ninth aspect, the opposing pattern is formed of a conductive paste.
According to the constitution of the eleventh aspect, in addition to the operation of the ninth aspect, the opposing pattern can be easily formed on the insulator layer.
The invention of a twelfth aspect is characterized in that in the constitution of the ninth aspect, the insulator layer is formed of a dry film of an epoxy resin having photosensitivity.
The invention of a thirteenth aspect is characterized in that in the constitution of the ninth aspect, the insulator layer is formed by curtain coat coating of a liquid epoxy resin having photosensitivity.
The invention of a fourteenth aspect is characterized in that in the constitution of the ninth aspect, the insulator layer is formed by printing using a photo-curing resist.
The invention of a fifteenth aspect is characterized in that in the constitution of the ninth aspect, the insulator layer is formed by printing using a thermosetting resist.
According to the respective constitutions of the twelfth to fifteenth aspects, in addition to the operation of the ninth aspect, the insulator layer can be easily formed so as to cover the signal transmission pattern.
The invention of a sixteenth aspect is characterized in that in the constitution of the ninth aspect, the opposing pattern is connected to a solid pattern provided on the other surface of the base material through a via hole for conducting electricity to pass through both the plane surfaces of the base material.